Various aquatic creature repellant devices, particularly shark, have been proposed over the years. A large number of these devices appear to rely on the electro-perception of sharks which is performed by the ampullae of Lorenzini in the shark's nose and head. These sensory organs have been shown, for instance, by Dr. Adrianus J. Kalmijn, to be extremely sensitive to electrical fields in sea water. Using fields decreasing to 5 nVcm-1 at distances 24 cm to 30 cm from the field source, Kalmijn was able to stimulate feeding attacks in response to electric fields simulating prey. It should be appreciated that this research was conducted against the background that the human body, especially when the skin is damaged, creates substantially stronger bioelectric fields, which some sharks in the ocean can detect from distances up to at least 1 m. The galvanic fields of metallic objects are usually even stronger, which, according to Kalmijn, would explain much of the aberrant behavious of sharks in the presence of man and underwater gear.
Therefore, shark repulsion maybe achieved by overstressing their natural electro-sensors. The so-called ampullary receptors in sharks' physiology may allow them to sense electromagnetic radiations of preys' heartbeats, for example. Bandwidth of those sensors is within the Extremely Low Frequency range (<300 Hz) of the electromagnetic spectrum.
U.S. Pat. No. 5,566,643 defines a method and apparatus for controlling aquatic animals, particularly sharks, wherein electrodes are immersed in a body of water and an electric field is created between the electrodes by applying electrical pulses thereto. This patent elaborates about generating a current in salted water, whereas literature makes it clear that ampullary receptors are sensitive to electric fields rather than current flow. This U.S. patent utilizes, among other things, electrodes that are in direct contact with sea water (their device was tested with 1 ohm shunt resistance across electrodes to simulate sea water conditions), which imposes to implement current limiting in the circuitry to minimize current consumption. The apparatus is immersed in water with direct electrical contact with the radiating electrodes. In addition, a current sensor must be used to limit the current, otherwise the supply source would be depleted quickly. Two immersed electrodes with a given voltage gradient, generate a current flow that is a direct path between the electrodes. This occurs because the resistance of the water is of very low impedance and that current seeks for least resistance, hence this is direct path. In addition, U.S. Pat. No. 5,566,643 discloses the use of a fixed pulse frequency, therefore the pulse frequency does not change over time.
The effect of repulsion is actually achieved by creating a pulsed electric field where the pattern meets a shark's receptors bandwidth and respects ionic properties of salted water. Literature suggests that the ampullary receptors have high sensitivity and respond to low frequencies in the 0.1-20 Hz range. As for the ions in salt water, it is disclosed in U.S. Pat. No. 4,211,980 that when an electrical field is created by two electrodes with a voltage gradient, the transient state consists of ions moving towards their respective opposite electrode (positive ions go to cathode and negative ions go to anode). When the system reaches a steady state, all ions stop moving. At this point, there is a barrier that inhibits the initial electric field magnitude. If this state is reached, a voltage gradient between electrodes is not effective.
With further reference to U.S. Pat. Nos. 5,566,643 and 4,211,980, these specifications have limited system considerations such as energy budgeting. In particular, they propose descriptions of transducers without considering the systems in their integrity. The power supply is a black box component and no strategy is proposed to make the apparatus well integrated in an application and convenient to use. In addition, power supply electronics are not described and so critical parameters such as supply autonomy are not addressed. In particular, issues such as how to recharge on-board local source of energy are also not addressed, which, in general, would be very important for the apparatus to sustain operation for long periods of time.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.